There is known a sequential gas carburizing apparatus in which a work such as an automobile part is carried through a carry-in chamber, a carburizing chamber and an oil tank chamber in this order for performing carburizing treatment thereon, and also the present applicant has disclosed, for example, Japanese Patent Application Laid-open No. H11-1759 (Patent Document 1).
[Patent Document 1]
    Japanese Patent Application Laid-open No. H11-1759.
In such a sequential gas carburizing apparatus, in order to avoid unevenness in carburization due to differences in positions of works in a furnace, disturbance in furnace atmosphere due to flowing-in of the outside air into the furnace, or risk of explosion or the like due to flowing-in of the outside air, a transforming gas which is of approximately five to ten times the furnace capacity per hour is supplied into a carburizing chamber to thereby adjust the pressure in the carburizing chamber to a predetermined furnace pressure. In this case, for example, an entrance side exhaust path is provided in a carry-in chamber connected to the entrance side of the carburizing chamber, and an exit side exhaust path is provided in an oil tank chamber connected to the exit side of the carburizing chamber, so as to supply the transforming gas to the carburizing chamber. At the same time, the inside atmosphere is exhausted from both the entrance side and the exit side of the carburizing chamber, thereby burning the exhausted gas outside.
In the carburizing chamber, a preheating zone, a carburizing zone, a diffusing zone, a hardening heating zone, and so on are provided in order from the entrance side to the exit side, and among them, the hardening heating zone is kept at a lower temperature compared to the other zones. For example, in general, the preheating zone, the carburizing zone, the diffusing zone are kept at approximately 930° C., whereas the hardening heating zone is kept at approximately 850° C.
On the other hand, in the sequential gas carburizing apparatus, the inside of the carburizing chamber is kept at a predetermined carbon potential (CP) of 0.8% for example. However, the CP (carbon potential) tends to increase when the temperature of the inside atmosphere drops, and thus the CP increases when the inside atmosphere moves from the carburizing zone or the diffusing zone kept at approximately 930° C. to the hardening heating zone kept at approximately 850° C. for example. For example, the CP of approximately 0.8% in the carburizing zone or the diffusing zone which are kept at approximately 930° C. increases to approximately 1.0% in the hardening heating zone which is kept at approximately 850° C. Then, this increase of CP in the hardening heating zone makes the carburizing treatment on works unable to be controlled precisely.
Accordingly, the inside atmosphere of the entire carburizing chamber is kept at a target value (0.8% for example) as much as possible by exhausting the inside atmosphere of the carburizing chamber to the entrance side of the carburizing chamber as much as possible, and thereby making the exhaust amount to the exit side (hardening heating zone), where the CP increases by temperature drop, of the carburizing chamber relatively small. For example, by exhausting 70% to 80% of the inside atmosphere in the carburizing chamber to the entrance side, and exhausting the remaining 30% to 20% to the exit side, increase of the CP in the hardening heating zone is suppressed effectively, which allows keeping of the CP in the entire carburizing chamber almost at the target value (0.8% for example) regardless of positions, and thus the carburizing treatment becomes precisely controllable also during the hardening heating.
For this purpose, conventionally, an operator appropriately adjusts the opening degrees of the entrance side exhaust path and the exit side exhaust path so as to allow exhausting of the inside atmosphere in the carburizing chamber to the entrance side and the exit side by a predetermined ratio (7:8 to 8:2). In this case, the operator sees for example the size of flame or the like, which is exhausted to the outside of the furnace and burnt, and adjusts the exhaust amount from the entrance side exhaust path and the exhaust amount from the exit side exhaust path, thereby controlling the inside atmosphere in the carburizing chamber to be exhausted by the predetermined ratio to the entrance side and the exit side.
However, such conventional control often relies on experiences and/or intuitions of an operator, and thus objective judgment is not possible. Also, since an experienced operator is required for operating the apparatus, a human-related load becomes large, and hence reduction of labor cannot be realized.